Modern data processing systems generally provide visual output by way of such output devices as cathode-ray tubes (CRTs). The performance of such systems can be enhanced when the visual display can receive information from more than one source, and which can overlay the information received from a first source (background) with the information received from a second source (foreground). The overlaying of such information can especially be useful where only a portion of the background information is hidden by the foreground information, as where the foreground information occupies a "window" of the display, and where the information hidden by the foreground window is not irretrievably lost.
It is especially useful in such systems to overlay the output of a data processing system with a dissimilar source, such as a digitized signal from a video camera. In such a system using a real-time source such as a video camera, it is preferable that the overlaying and display of the foreground information occur on a real-time basis, in order to take full advantage of the video camera input.
Referring to FIGS. 1a and 1b, prior methods for displaying images from multiple sources are illustrated. FIG. 1a illustrates the overlaying of information from multiple digital sources 2. The output of each digital source 2 can represent the video data in the well-known RGB (red-green-blue) format. Digital-to-analog converters 4 receive the RGB information from each of digital sources 2, and convert the digital RGB information into analog signals for each of the elements therein. The output of the DACs 4 is then received by an analog mixer 6, which selects among the signals from the multiple DACs 4 as required for the overlaying of information among the digital sources 2. FIG. 1b illustrates a second system, containing frame buffers 8 which store bit-mapped representations of the information to be displayed on the video output device. Each of frame buffers 8 communicate with a data source (not shown), such as a host processing unit or the digitized output of a video camera, by way of its parallel port 10; each of frame buffers 8 also have, as is well known in the art, a serial port 12 for communicating the bit-mapped data to the video display. Serial ports 12 are received by a digital mixer 14, which constitutes a multiplexing function (or another operation on the serial digital data received, such as a logical OR) on each of the digital values presented by serial ports 12 thereto. The output of digital mixer 12 is applied to a DAC 16, for providing the analog RGB output as in FIG. 1a for application to a video display device.
A problem in each of the prior methods shown in FIGS. 1a and 1b is the synchronization of data from the multiple sources. The data presented to each of the mixers (analog mixer 6 of FIG. 1a, and digital mixer 14 of FIG. 1b) from each of the sources must correspond to the same position on the video display, thereby requiring that the data from the foreground source must be presented at the same rate as, and synchronized with, the data from the background source. This places constraints on the data arriving from the foreground source, as the video display position of the foreground image must coincide with the position of the hidden portion of the background image. This requires that the foreground image be buffered, and thereby delayed, to coincide with the operation of the video display as controlled for the background image.
It is therefore an object of this invention to provide a system for overlaying a foreground image onto a background image, without requiring that the display positions of the two images be synchronized.
It is another object of this invention to provide such a system which allows the foreground image to be input from a real-time source, such as a video camera.
It is yet another object of this invention to provide such a system which allows the overlaying of the foreground image in selectable positions of the background image, such selection being transparent to the receipt and transmission of the foreground image.
Other objects and advantages of the instant invention will be apparent to those of ordinary skill in the art having reference to the following specification in conjunction with the drawings.